Why isn't this more widely used? First, we have to understand that digital painting is a fundamentally different beast than traditional, in how you can use the tool, what you need to know, and how you learn it.

In traditional, you tend to have some rules that make it easy to get good colors.
One method is to restrict your paints to a few primary colors and burnt sienna (or another low saturation brown).
Then by mixing these colors you get the colors you want, in a way where these colors have a good relationship with the others.
So a simple rule is for instance that you get your green by putting lemon yellow with some black and a bit of sienna to lower the saturation.
The relationship is almost 'free' from the properties of the physical pigments if you follow such simple rules.

What's the deal about 'relationship'? Colors are not treated individually, they are perceived in context.
In real life, lightning will unify a scene by changing colors. However, there is a semantic superposition that affects it.
Grass is green in your mind, so if the color you see is a light blue/purple, the brain will adjust its understanding to match it.
This is where many illusions come from (most famously the white and gold/blue and black dress meme come from).

This is a big deal to learn painting, but can be almost completely side-stepped by learning how to mix colors.

In digital however, artists got used to the fact that color is generally treated as HSV and adjusted their approach.
So the most commonly taught approach to manage colors is to work in black and white, only dealing with value, which is the most essential component of color.
Values are the primary issue because this is fundamentally what implies to our brain what are the normal vectors of light, and so carry almost all 'volume information'.
Then, you can apply colors via some layers set to multiply, color and overlay mode.
Note that it has some antecedent in the traditional world, called 'grisaille' technique.

So, you can color pick anywhere you want on the color wheel without mixing. The impact of color mixing is actually pretty small.
The other technique is called gamut masking; you're only allowed to pick colors on a subset of the color wheel that is in a good HSV relationship with the chosen color scheme.
The thing is, even though this is not perceptually accurate, this is good enough.

The main impact of color mixing is that if working with low opacity brushes, the colors will look 'muddy'.
This problem is mostly side-stepped by avoiding it altogether with high-opacity brushes, and using other layer blending modes.

Because now most digital artists have learnt through these techniques, bringing techniques that work best coming from the traditional world is not very useful, to say the least.
In my own practice, traditional and digital are indeed almost entirely separate activities, at every level.
The fundamental, theoretical understanding of all aspects is the same, but it doesn't really matter very much; almost all other artists I ever discuss with do not have such an understanding, and build their practice from a more 'human'-centric foundation (technique and artistic concepts).

I'm of course interested in the space, and it is a bit similar to asking why would people would use bash or plain text; it's not necessarily that these are the best tools, but there is much more to it.